Macrophages play a central regulatory role in many aspects of hematopoiesis. We propose to continue investigation of the production of colony-stimulating factors of different types by macrophage subsets, with particular emphasis on the production of factors promoting neutrophil (G-CSF) and pluripotent (mixed) colony formation in vitro. The regulatory role of macrophages in supporting long-term cultures of pluripotent stem cells will be investigated, with particular emphasis on production of hematopoietic enhancing factors by lipid-laden, marrow-derived macrophages. Vitamin D and its analogs have been shown to play a role in inducing myeloid leukemia cell lines to differentiate to macrophages, and we are exploring the role of this vitamin in normal macrophage development and function.